1. Field of the Invention
This invention relates generally to multiple-speed automotive vehicle transaxles for use with internal combustion engines. More particularly it pertains to a transaxle having a planetary gear set transfer drive mechanism disposed parallel to multiple-speed gearing.
2. Description of the Prior Art
A transaxle and an engine for an automotive vehicle usually are assembled in a common powertrain package with the axis of the engine crankshaft parallel to the transaxle output shaft axis. The crankshaft axis and the transaxle output shaft axis are transversely mounted in the vehicle in a forward powertrain compartment. The output shaft drives the steerable front wheels of the vehicle through universal joints and halfshaft assemblies.
Examples of transaxles of this kind are shown in U.S. Pat. Nos. 4,607,541 and 4,418,585. Each of these patents describes a pair of simple planetary gear units arranged to define a compound torque flow path with selectively engageable clutches and brakes for establishing the driving speed ratios. The planetary gearing, like the planetary gear arrangement of the present invention, is coaxial with the output shaft axis, or it is parallel to the output shaft axis. That parallel axis is coincident with the hydrokinetic torque converter axis situated between the torque input elements of the gearing and the engine.
Each of these prior art reference patents describes a transaxle capable of achieving four forward driving speed ratios and a single reverse speed ratio. Each of them also describes a hydrokinetic torque converter having a lockup clutch for effecting a mechanical driving connection between the impeller and turbine of the converter.
Because of the constraints that are imposed on an engine powered vehicle driveline by the limited number of ratios available in the planetary gearing, it is not possible with such four speed ratio transaxles to achieve the full performance potential and the full potential for fuel economy that would be available if five or more ratios in the driving range were available.
We are aware of recent attempts to modify existing four speed ratio transmissions to achieve five driving ratios in an automotive vehicle driveline, but these attempts usually involve the use of auxiliary gearing at the torque output portion of the basic four speed ratio gearing. They also include separate clutches and brakes for controlling the driving ratio of an auxiliary transmission. An example of a driveline of this kind may be seen by referring to Ward's Engine Update, an automotive news magazine, dated Jul. 15, 1989, page 6.
Such modifications to normal four speed ratio gear systems have only limited potential for improving efficiency and performance because of the limitations on the ratio spread available by use of auxiliary planetary gearing. Those limitations are imposed by the geometry of the auxiliary planetary gearing itself since the change in ratio range that is available by reason of the addition of the auxiliary gear unit is limited by the available range of pitch diameter ratios of the internal gear and the external gear of the auxiliary planetary gear system. Further, the use of such an auxiliary gear system adds to the overall length of the transmission, which makes it difficult to assembly the transmission in a compact engine and transmission compartment. In most vehicle applications, especially front wheel drive vehicles, styling considerations impose limitations on available space for the powertrain package.